A quantitative autoradiographic method for the determination of local rates of protein synthesis in brain in vivo has been developed in which a carboxyl-labeled, aliphatic, branched-chain amino acid is used as the tracer. A four-compartment model for the behavior of the tracer amino acid in brain has been analyzed, and an equation has been derived that defines the rate of incorporation into protein in terms of the time course of plasma amino acid specific activity, final tissue level of 14C, and lambda(i), the steady state ratio of the distribution volumes for the labeled and unlabeled amino acid in the immediate precursor pool for protein synthesis. lambda(i) is a measure of the fraction of the amino acid in the precursor pool for protein synthesis that is derived from plasma; the remainder is derived from protein degradation. The value of lambda(i) for the brain as a whole in conscious, adult, male rats has been determined to be 0.58 and 0.35 for leucine and valine, respectively. Results of studies of lambda(i) for leucine in different brain regions and under various experimental conditions show that the fraction of the amino acid precursor pool coming from protein degradation is not uniform in all brain regions and can change under certain conditions. "Flooding" with L-valine results in a 100% increase in the value of but no effect on the rate of incorporation of valine into protein (lCPSval) in the brain as a whole. Studies carried out on applications of the method that examine some basic neurobiological questions include: 1) barbiturate & ketamine anesthesia; 2) electrical stimulation; 3) regeneration ; 4) chronic cocaine administration; and 5) hibernation in ground squirrels.